Milling tools for chip removing machining of, above all, workpieces of metal (steel, aluminium, titanium, etc.) are generally composed of a rotatable basic body or milling cutter body, most often of steel, as well as a plurality of replaceable milling inserts of cemented carbide, ceramics or the like. Because the milling inserts are articles of consumption as a consequence of the fact that they are worn fairly fast, it is most often desirable to form the same with as large a number of cutting edges as possible. For this reason, the milling inserts may be made double-sided so far that the underside is formed with the same number of cutting edges as the upperside, while doubling of the number of cutting edges in comparison with single sided milling inserts. Therefore, milling cutters for face milling are often equipped with milling inserts that are double-sided and have a quadratic basic shape with four cutting edges, i.e., four pairs of co-operating main edges and secondary edges, along the upperside as well as the underside, and which are mounted at an effective setting angle of approximately 45° in the milling cutter body. In such cases, the main edge and the secondary edge form an angle of 135° with each other.
Problems forming the basis of the present invention are associated with double-sided face milling inserts of the type that is denominated “negative”, and which are formed with clearance surfaces that extend perpendicularly to the neutral plane to which the uppersides and undersides are parallel. In order to provide requisite clearance between, on one hand, the clearance surface rotationally present behind the active, surface-wiping secondary edge, and, on the other hand, the generated, plane surface of the workpiece, the milling insert is mounted with a negative axial tipping-in angle in the milling-cutter body. At the same time, the milling insert is also given a negative radial tipping-in angle in order to provide clearance between, on one hand, the clearance surface behind the chip-removing main edge, and, on the other hand, the generally cone-shaped surface generated by the same. The negative axial tipping in of the milling insert gives rise to, on one hand, greater axial cutting forces than those cutting forces that arise when the milling inserts are positively tipped in, and, on the other hand, difficult-to-master problems with the chip formation as well as the chip evacuation, among other things so far that the chips tend to be directed obliquely downward toward the generated plane surface rather than escaping therefrom.
Before the background of the invention is additionally described, it should be pointed out that certain fundamental concepts found in this document, e.g., “clearance angle”, can be of either a nominal or an effective character. When, for instance, a clearance angle is “nominal”, the same only relates to the milling insert as such, i.e., without conjunction with the milling cutter body, but if the same is “effective”, reference is made to the clearance angle that occurs when the milling insert is mounted in the rotatable milling cutter body to undertake chip removal.
The problems caused by the negative axial and radial tipping-in, respectively, are particularly accentuated in milling cutters having older type milling inserts, which have a prismatic basic shape and include cutting edges, the main edges of which are straight and pair-wise parallel along their common clearance surfaces, as are also the secondary edges straight and pair-wise parallel along the common clearance surfaces thereof. In this case, the main edges become particularly blunt-cutting and give rise to pronounced chip formation and chip evacuation problems, because they have to be tipped in at a great negative axial angle to give the desired, efficient clearance angle between the generated plane surface of the workpiece and the clearance surface behind the surface-wiping secondary edge.
More recently, a number of proposals of solutions of the above-mentioned problems have appeared. Thus, in U.S. Pat. No. 5,807,031, a double-sided, quadratic face milling insert is disclosed, the chip-removing main edges of which are nominally inclined in relation to the neutral plane of the milling insert, more precisely in such a way that the individual main edge, counted from a first end adjacent to the co-operating secondary edge, first declines toward the underside of the milling insert and then, from a lowest point, again rises toward an opposite end. In such a way, the above-mentioned problems are solved in a general way, so far that the effective axial angle of the proper main edge is reduced from a relatively great negative value to a smaller, more positive value, in spite of the milling insert, per se, (i.e., the neutral plane of the milling insert) having a sufficiently great negative axial angle in order to provide the requisite clearance behind the surface-wiping secondary edge. However, this known milling insert is, nevertheless, associated with a number of shortcomings and disadvantages. One such disadvantage is that the clearance surfaces of the secondary edges—like the corresponding clearance surfaces of the older milling inserts—are plane and extend perpendicularly to the neutral plane of the milling insert. This means that the milling insert, to give a sufficient effective clearance immediately behind the active secondary edge, still requires an axially negative tipping-in angle that is fairly great. Another disadvantage is that the two secondary edges along a common clearance surface at each corner of the milling insert are mutually parallel. This means that the transition between the individual secondary edge and the appurtenant main edge forms a fairly sharp or “bobish” corner (as viewed nominally in side view), so far that the angle between the secondary edge and the declining main edge is considerably smaller than 180°. Thus, in the preferred embodiment, this angle amounts to 165°-170°. Because the corner transition between the main edge and the secondary edge is the part of the milling insert that is absolutely most exposed to, among other things, forces, heat and erosion, the fairly distinct bob on the same means that the milling insert becomes fragile and gets a limited service life. In addition, the wear of the bob gives easy rise to visible stripes in the completed, wiped-off surface, more precisely in the form of per se shallow, but nevertheless most detrimental grooves in the surface being plane in other respects. In other words, the finish of the generated surface becomes rather mediocre.
A face milling insert which resembles the milling insert described above and which essentially is impaired with the same disadvantages as the same is previously known by U.S. Pat. No. 7,306,409.
The present invention aims at obviating the above-mentioned disadvantages of the previously known face milling inserts and at providing an improved face milling insert.
An object of the invention to provide a double-sided face milling insert, the axially negative tipping-in angle of which in the milling cutter body is reduced to an absolute minimum while optimizing the performance of the milling tool.
Another object of the invention is to provide a face milling insert, in which the desired, cutting-technical improvements can be attained without the milling insert being weakened by unnecessary reduction of the amount of material (cemented carbide) in the same.
In addition, the design of the milling insert should offer the possibility of adapting the clearance under the secondary edge to specific applications in which the machined material requires an enlarged clearance, without the mounting of the milling insert in the milling cutter body being affected.
Yet another object of the invention is to provide a face milling insert that can be manufactured by pressing and sintering of, e.g., cemented carbide powder, without the design of the milling insert making the process more difficult and/or detrimentally affecting the surface quality of the milling insert. Most suitably, the milling insert should be possible to be directly pressed, i.e., obtain the final shape thereof without grinding.